For older versions of GPDB, convert string to vectors before validation
of the arguments

This fixed the bug where data columns will be reported as missing.
Elastic Net regularization for linear models

FISTA and IGD optimizers are supported

Grouping support will be added later.
Elastic Net regularization for logistic models

Grouping support will be added later
Set use_active_set default to be false

Update user doc

Better help messages
LDA: make get_perplexity faster
Added a check for NULL values in array in the C++ abstraction layer

If there is NULL, throw an error message.